Michigan it’s actually illegal to include basements in the square footage when you list or advertise your house, even if the basement is finished. This is probably a big part of the difference.

Another factor is the age of the housing stock. Older states, where most of the homes were built long ago, tend to have smaller houses, in my experience. Whereas states that have grown significantly in more recent years tend to have more recently-built homes, which also tend to be larger. Utah is definitely in the latter camp.

Large Utah families and polygamist families are probably a factor too.